The invention relates to an electrically weldable spring hinge including a housing with a cavity therein for receiving a hinge element.
Spring hinges of this type are known. They are used as hinges for ear pieces of spectacles and are fastened to the ear pieces of the spectacles by means of an electric welding process. During the electric welding, the spring hinge is pressed firmly against an ear piece of the spectacles. The applied pressure is within a range of, for example, 70 to 700 N. As soon as the pressure forces are applied, a welding current of 2000 A to 8000 A is conducted via the housing and the ear piece. Welding projections are provided in the contact region between the spring hinge and the ear piece of the spectacles, which heat up and melt at the high currents. This produces a firm connection between the spring hinge and the earpiece of the spectacles. It has been found that, at the high pressure forces, deformation of the housing of the spring hinge, in particular of the cavity provided inside the housing, may occur. This cavity accommodates a hinge element. The latter is displaced inside the cavity during use of the spring hinge. During the electric welding, material displacements which impair optimum functioning of the spring hinge may also occur: it is possible for material to be displaced into the interior of the housing, namely into the cavity, so that a protrusion is produced there. If deformation of the housing and thus of the cavity takes place or if material is displaced into the cavity during the welding, the mobility of the hinge element relative to the housing is impaired and thus the functioning of the spring hinge is impaired. Such deformations of the housing occur in particular in small spring hinges, which are being increasingly used.